Hi there! Great to see you again on our online course EASY Geosteering!

In this course, we simplify the key geosteering aspects, breaking them down into easy-to-understand concepts.

This article contains the full transcript of a video lesson also available on YouTube:

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Course structure

We are currently in the second part of the course — BHA components, and in the previous video we covered RES measurement.

And today, we’ll continue with neutron-density logging.

Let’s get started!

Density and porosity

The third type of logging is density and porosity, and in fact, a single downhole tool performs two different measurements.

To carry out these measurements, we use artificial radioactive sources, which include two components: Cesium one-three-seven and Americium two-forty-one Beryllium.

The first is a source of gamma radiation, which helps us detect density and record borehole images.

The second is a source of fast neutrons, allowing us to measure hydrogen content, which is then recalculated into porosity.

Neutron-density logging is extremely informative and it is used for lithological differentiation, saturation and permeability calculations.

How does it work?

While BHA makeup, MWD engineer loads the sources into the tool, and the proper execution of this procedure critically affects the logs quality.

This type of measurement has the smallest depth of investigation — only a few cm — so it is crucial to minimize the gap between the tool and the wellbore surface, especially for density measurements.

For this reason, the tool often includes a stabilizer, where windows are positioned for the GR source and its detectors.

So, while drilling Cs emits gamma rays into the formation, allowing them to interact with it. By analyzing the number and energy levels of gamma rays that return to the detectors, we can estimate the formation’s density.

In turn, Americium Beryllium emits fast neutrons, which also interact with the formation. Due to their nature, neutrons primarily interact with hydrogen in pore spaces. During these interactions, fast neutrons lose energy, slow down, and become thermal neutrons.

These particles can be captured and counted by the helium neutron detectors. By analyzing the number of thermal neutrons, we can estimate the formation’s porosity.

Compatible scales

Before we proceed with readings in common formations, let’s consider compatible scales — an important and unique aspect of neutron-density log interpretation.

For GR and RES logging, there are no strict rules regarding scale selection, but for neutron-density, we should use a specific scale.

When drilling in a sandstone formation, we typically set the left scale for density at 1.65 and the right scale at 2.65.

And now, pay attention — porosity uses an inverse scale, with 60 on the left side and 0 on the right.

Why is it so?

It is done for convenient visualization. The point is, if we drill in a clean and non-shale formation filled with water with density of 1 gram per cubic cm, the density log and porosity log will plot on the same part of the track and overlap with each other.

If we encounter gas in the formation, we will observe curve separation or gas crossover — the density log will go down on the track, while the porosity log goes up. The root cause is that gas has lower density and lower hydrogen content per unit volume than water.

If we encounter a shale formation, we will observe an inverse crossover — the density log will go up, while the porosity log goes down, since shale has high density and high porosity.

So, this is compatible scales for sandstone formations, but for other formations, there are different scales. For example, for limestone, you should use the following: for density, from 1.95 to 2.95, and for porosity, from 45 to -15. Negative values have no physical meaning, just remember, this is done for convenient and quick log interpretation.

Density and porosity readings in common formation

Ok, let’s add neutron-density to GR and RES on our log to form triple-combo in RT.

Here we have reference value for density: greater than 2.3 grams per cubic cm indicates non-reservoir, while density lower than 2.3 suggests pay zone.

There is no reference value for porosity here, but in some projects, you might encounter one.

So in the first formation we observe shale effect — high density and high porosity. And as we already know shale has low resistivity and high GR.

In the second formation we observe gas effect and inverse crossover — low density and low porosity. This interval has the highest RES and low GR.

In formations 3 and 4, the neutron-density readings are the same, and the curves overlap.

This indicates that formation 3 is a clean, non-shale reservoir filled with oil, and this oil has a relatively high density, close to that of water.

Using the neutron-density log, we cannot detect the water-oil contact, meaning that RES is the only one reliable method for this.

Meanwhile, detecting the gas-oil contact using RES is not always possible, because gas and oil within the same formation can sometimes have similar RES values. But, neutron-density log can usually detect it with high accuracy.

As we can see different types of logging complement each other, which is why the triple-combo service provides highly informative data.

And here we need to make an important note — of course in nature there are enormous number of different formations and geological conditions.

For example, sometimes it’s impossible to detect Gas-Oil contact even if we have triple combo in RT; sometimes we encounter argillite, which has low GR level like sandstone, sometimes we have shale with high RES and so on.

That’s why it is critically important to familiarize yourself with regional geology and find out how specific formation types appear on offset well logs.

Ok let’s return to hard layers and coals– now we can clearly differentiate them.

So carbonate sandstone has high density and low porosity — both curves go up on the track.

Coal has low density and high porosity, curves go down.

However, like shale, coal cannot serve as a reservoir due to its low permeability, despite the fact these formations have high porosity.

Alright, we’ve covered the neutron-density logging. Hope you liked this article! Please feel free to leave your thoughts in the comments and don’t forget to follow my profile for upcoming lessons!